I am not sure but they probably supply roughage as plant fiber provides that to us. Fish were built to eat fish and scales are a part of fish so I am sure they evolved to use them along with the bones. There are a lot of bones in a whole fish and that is where fish obtain calcium from.
We humans take vitamins because much of our diet now is not a normal human diet.
If we reverted to hunter gatherers again, we would probably not need vitamins as we evolved to eat what is available freely. Ancient man did not eat much Hostess Twinkees, Cocoa Puffs, Coca Cola, SPAM, fuzili, Cheese in a can, hot dogs, M&Ms etc.
They ate mostly fruits, roots and a little meat and fish if they could get it. That food was un processed and they probably ate the entire animal, guts and all (owwweee)
